1. Field of the Invention
This invention relates to pressure feeding systems and more particularly to a float suitable for use in a system for pressure feeding of slurry.
2. Description of the Prior Art
A system for pressure feeding of slurry of the prior art will be described by referring to FIG. 1, wherein the numerals 1-3 designate supply chambers, and the symbol T designates a slurry tank storing a slurry introduced into the supply chambers 1-3 by a slurry pump SP. The slurry introduced into the supply chambers 1-3 is discharged therefrom through discharge pipes 7-9 respectively.
Slurry supply pipes 4-6 supplying slurry from the slurry tank T to the slurry chambers 1-3 mount check valves B.sub.1 -B.sub.3 respectively, and the slurry discharge pipes 7-9 mount check valves D.sub.1 -D.sub.3 respectively. A driving liquid is stored in a driving liquid tank OT and introduced by a high pressure pump LP into the supply chambers 1-3 through pipes 10-12 respectively. The driving liquid introduced into the supply chambers 1-3 is returned through pipes 13-15 to the driving liquid tank OT respectively.
Change-over valves A.sub.1 -A.sub.3 are mounted in the pipes 10-12 and change-over valves C.sub.1 -C.sub.3 are mounted in the pipes 13-15 respectively. The symbols F.sub.1 -F.sub.3 designate floats located in the supply chambers 1-3 respectively and floating in the interface between the driving liquid and the slurry therein. Sensors SH.sub.1 -SH.sub.3 and SL.sub.1 -SL.sub.3 are for sensing the upper and lower limits of the floats F.sub.1 -F.sub.3 respectively.
The manner in which the slurry is fed from one supply chamber will be described. As shown, the supply chamber 1 is filled with the driving liquid. Actuation of the slurry pump SP with the check valve D.sub.1 in the slurry discharge pipe 7 and the change-over valve A.sub.1 in the pipe 10 being closed and the change-over valve C.sub.1 in the pipe 13 being open supplies the slurry from the slurry tank T to the supply chamber 1 by opening the check valve B.sub.1 in the slurry supply pipe 4 and discharges the driving liquid in the supply chamber 1 through the change-over valve C.sub.1. This causes the interface between the driving liquid and slurry or the float therein to rise. Upon the sensor SH.sub.1 sensing the arrival of the float F.sub.1 at its upper limit, the change-over valve C.sub.1 is closed and the change-over valve A.sub.1 is opened. Actuation of the high pressure pump LP opens the check valve D.sub.1, and the driving liquid presses the slurry and urges same downwardly, so that the slurry is forced out of the supply chamber 1 into the discharge pipe 7. By actuating the supply chambers 1-3 in chronological sequence in accordance with a suitable timetable, the slurry can be continuously fed under pressure from the supply chambers 1-3.
In the aforesaid system for feeding the slurry under pressure, the pressure in each of the supply chambers 1-3 reaches several kg/cm.sup.2 when the slurry pump SP is actuated to fill the slurry chambers with the slurry, and the pressure in each of the supply chambers 1-3 reaches several scores of kg/cm.sup.2 when the high pressure pump LP is actuated to feed the slurry in the supply chambers under pressure to its destination. This makes it necessary to fabricate the floats F.sub.1 -F.sub.3 such that they can withstand changes in pressure. It has hitherto been usual practice to form the float in hollow, sealed spherical construction and to design the specific gravity of the float to give to it an apparent specific gravity, or total weight per total volume of float, which is midway between the slurry and that of the driving liquid. This type of float is capable of withstanding an external pressure when the pressure is low. However, when the pressure rises above a certain level, difficulties are experienced in imparting to the float enough strength to withstand an external pressure with this apparent specific gravity. Meanwhile the apparent specific gravity would become too high if the thickness of the float is increased to enable it to withstand the high external pressure. Thus the maximum pressure that the floats of the prior art have been able to withstand could be about 50-60 kg/cm.sup.2.
To this problem, proposals have been made to fill a charge of liquid in the float and to attach a bellows thereto to equalize the pressure inside and outside the float. However, this has not made it any easier to obtain a float with a predetermined apparent specific gravity. For one thing, the liquid to be filled in the float should be low in gravity, but such liquid is not readily obtainable. For another, if the float is covered with a shell of metal, then it is inevitable that the float has a substantial weight.